An electrocardiogram is a test that graphically records the electrical activity of the heart. The electrocardiogram or ECG (sometimes called EKG) is used worldwide as a relatively simple way of diagnosing many heart conditions. It records the small electric waves being generated during heart activity using body surface electrodes attached to a patient. The electrodes are placed in a particular pattern for clinical use because electrical signals generated by a human heart appear in a characteristic pattern throughout the body, and on its surface subject to their position.
A procedure developed by Willem Einthoven in 1901 inter-related three electrodes specifically oriented on the body (right arm, left arm, and left leg). These electrodes are at the apices of a physiological triangle known as Einthoven's triangle, as shown in FIG. 1a. The difference in electrical potential between the left and right arms is designated lead I; lead II is the difference in electrical potential between the left leg and right arm; and lead III is the difference in electrical potential between the left leg and left arm. Thus, the Einthoven triangle resembles a triangle standing on its tip “▾.”
These electrodes provide bipolar recordings of the voltage differential between two electrodes. By convention, the positive electrode is placed on the left arm, with the negative electrode on the right arm. In the lead II configuration, the positive electrode is on the left leg and the negative electrode is on the right arm. Lead III has the positive electrode on the left leg and the negative electrode on the left arm. The limb leads can be attached to the end of the limb (wrists and ankles) or at the origin of the limb (shoulder or upper thigh). The difference in electrical potential between two of the electrodes constitutes the signal.
Referring to FIG. 1b there is shown a simplified illustration of a conventional electrocardiograph 100 in place on a patient. The ECG 100 requires at least three leads (therefore three electrodes are needed). These three electrodes are applied one on each of the patient's arms 110 and 112. The third electrode 120 is applied on the patient's left leg.
A fourth electrode 140 is placed on the patient's right leg as an electrical ground. The ground can be at other locations on the body but at a reasonable distance from the other electrodes to ensure a good signal. In addition, there are six precordial (chest) leads 160 designated V1-V6 (not shown here), for a total of twelve leads. Their conventional placement is illustrated in FIG. 1c. 
The electrodes are easy to apply and this conventional placement of electrodes works well in a hospital setting and in a doctor's office. The problem arises, however, when it is desirable and sometimes necessary for an ECG to be used outside of a conventional medical setting. For example, a patient with chronic heart problems may want to have a portable ECG in the home or the office. Airlines may find it necessary to have a portable ECG in airplanes for in-flight emergency use. The signals produced by a portable unit can be transmitted to a doctor on the ground who can then interpret the signals and advise the airline staff as to whether to use an on-board defibrillator.
Electrodes must be positioned in an anatomically correct pattern so that the readings are valid. One problem with this conventional electrode placement is that leg electrodes are not conducive to portability.